Ask the Vet

Photosensitization Tied to What Cattle Eat

Light-colored cattle are more susceptible to photosensitization. (DTN\Progressive Farmer photo by Victoria G. Myers)


We had a Charolais cow with large patches of skin on her head and back, that are scabbed over and look like they are peeling up around the edges. She is in good body condition and does not seem sick. What could this be?


This sounds like photosensitization. I have described it in this column before as a chemical sunburn. It is more common on light skin and mucus membranes.

There are two forms of photosensitization. Primary photosensitization is due to consumption of a plant that contains photodynamic agents. These are absorbed into the blood stream and travel to the skin. Ultraviolet (UV) light activates them and creates a severe inflammatory reaction in the skin. Plants most commonly associated with this condition include St. John's Wort, buckwheat, and smartweed.

Secondary photosensitization is far and away the most common form we see. With this condition, the liver is damaged from something the animal ate or some type of liver disease. Chlorophyll from plants the cow eats is broken down into phylloerythrin and excreted by the liver in bile. With liver damage, phylloerythrin builds up in the blood stream and the skin. It is activated in the skin by by UV light, creating skin lesions. Plants most associated with this form include alsike clover, comfrey, rattle box, fiddleneck, and ragwort.

Treatment involves removing animals from the pastures containing the plants causing the problem and protecting them from sunlight. If the teats of cows are affected, they may not allow calves to nurse. In those cases calves will need supplementation until the cows recover. In some cases, antibiotics may be needed if skin becomes infected.